The present invention relates to a drive assembly in a boat, comprising a propeller drive which is arranged in a fixed manner on the outside of a boat hull and has an at least essentially vertical drive shaft which, via an angle gear enclosed in an underwater housing, drives in a counter-rotating manner a pair of at least essentially horizontal propeller shafts each with their own propeller, and a drive unit which is arranged on the inside of the hull and to which the vertical drive shaft is drivably connected.
It is a known fact that, in fast motor boats, it is possible to achieve considerably higher overall efficiency with an outboard drive with twin counter-rotating propellers coupled to an inboard engine than with an inboard engine coupled to a straight shaft with a single propeller. Until now, outboard drives in fast boats have with few exceptions been of the type which is suspended steerably as well as trimmably and tiltably in the transom stern of the boat. Such an exception is disclosed and described in SE 8305066-6, where a special embodiment of a drive with a pusher propeller and a tractor propeller is installed in a fixed manner and projects down from the bottom of the hull. The advantage of being able to trim the drive at different angles in relation to the transom stern of the boat is that the drive angle can be adapted to the position of the boat in the water, which depends on loading, speed and weather conditions, so that optimum propulsion can be achieved under different operating conditions. The advantages of being able to trim the drive are most apparent in smaller and medium-sized fast-moving boats up to about 40 feet. The larger and heavier the boat is, the less its position in the water is affected by said factors and the smaller the need to be able to trim the drive. At the same time, the cost of the drive increases considerably, the greater the power that it is to transmit. For these reasons inter alia, outboard drives are seldom used in boats in the size class over 40 feet, but in this case the engines drive straight propeller shafts with a single propeller via inboard-mounted reversing gears.
The object of the present invention is generally to provide a drive assembly of the type referred to in the introduction, which is primarily but not exclusively intended to replace a conventional inboard installation with reversing gear and a straight shaft in larger boats, and in this connection, compared with the inboard installation, to bring about not only higher overall efficiency and better performance but also simplified installation and lower installation weight.
According to the invention, this is achieved primarily by virtue of the fact that the propellers are tractor propellers and that the blade areas of the propellers are adapted to one another in such a manner that, at least under certain operating conditions, the aft propeller works in a cavity-generating manner whereas the fore propeller works in a cavitation-free manner.
It is previously known to use a propeller combination of a fore and an aft propeller together with steerable outboard drives, in which combination, at least at higher speeds, the aft propeller works in a cavity-generating manner whereas the fore propeller works in a cavitation-free manner. In this way, it is possible to reduce the grip of the propellers in the water slightly during turning, so that a certain sideways sliding occurs and uneven movement is avoided, which is essential as far as boats with a Froude number close on 5 are concerned. However, it has been found that such a propeller combination can lead to various advantages even in boats with a lower Froude number and with fixed drives with tractor propellers. When the aft propeller generates cavities, the resistance in the water from the underwater housing lying behind is reduced. Moreover, cavitation damage to the underwater housing is prevented when the aft propeller generates cavities. A further advantage is that the cavitation threshold is raised in the speed range, which means that the upper speed limit of the propeller can be raised or alternatively that the blade area can be reduced.
According to a development of the drive according to the invention, another possibility afforded by a drive with tractor propellers is the positioning of an exhaust discharge outlet in the aft side of the underwater housing, which means that it is possible inter alia to utilize the ejector effect which the water flowing past exerts on the exhaust gases streaming out in the same manner as when the exhaust gases are conveyed out through the propeller hubs. When the exhaust gases are conveyed out in the rear edge of the underwater housing instead of through the hubs, the hub diameter and thus the overall propeller diameter can be reduced, which is advantageous in a number of respects. On the one hand, the mass and the mass forces are reduced and, on the other hand, the space requirement under the bottom of the hull is reduced, which means that the drive shaft housing can be designed so as to be shorter and consequently lighter than if propellers with an exhaust discharge outlet in the hubs were to be used.